More coverage in COS CI upgrade tests

We currently have little coverage in our [COS integration tests](https://github.com/canonical/observability-stack/tree/main/tests/integration/cos). We could significantly improve coverage with 2 additional tests:
1. Coordinater charms `nginx` directive assertions
- We can access the nginx config file in the coordinator with `jssh --container nginx mimir/0 "cat /etc/nginx/nginx.conf"`
- e.g. [Mimir's config](https://pastebin.canonical.com/p/SrXqjDKcYY/)
- then curl each of these endpoints and ensure they route correctly, e.g. `request.code == 200`
2. otelcol pipeline assertion
- https://github.com/canonical/observability-stack/pull/170
- TL;DR, ensure there are no `ERROR` and (in some cases) `WARNING` logs in the internal otelcol logs
- i.e. no operational issues in COS since Otelcol is the central aggregator